Pulmonary vascular remodeling is a consequence of pulmonary hypertension that can lead to right ventricular failure. However, the mechanisms by which remodeling causes ventricular failure are unknown. Previous conventional thinking was that arteriolar muscularization led to reduced lumen diameters and increased steady ventricular work, but recent evidence has challenged that view. Our overarching hypothesis is that pulmonary vascular collagen accumulation is responsible for increased pulsatile ventricular work, which severely impairs function, and does so via arterial stiffening. In the systemic circulation, arterial stiffening has been recognized as an independent risk factor for cardiovascular mortality. The impact of arterial stiffening on right ventricular function represents a major knowledge gap in the field. Here, we seek to test the specific hypotheses that vascular collagen accumulation in response to pulmonary hypertension (1) increases pulmonary arterial stiffness, (2) increases pulmonary arterial pulse wave reflections, and (3) impairs right ventricular function. Furthermore, we seek to show that the return to normal arterial, hemodynamic and ventricular function during recovery is dependent on the degradation of collagen and that preventing vascular collagen accumulation (with a novel anti-fibrotic treatment agent) prevents progression. We will test our hypotheses with state-of-the-art biomechanical and hemodynamic experiments on wild type and genetically-engineered mice. We will also explore the clinical correlates of these endpoints in a clinical pilot study. Our specific aims are: 1. To determine the role of vascular collagen accumulation in pulmonary arterial stiffening - via pressure- diameter measurements in isolated pulmonary arteries and the pulmonary vascular network. 2. To determine the role of vascular collagen accumulation in pulmonary arterial wave reflections - via pulsatile pressure-flow measurements in isolated, ventilated and perfused lungs. 3. To determine the role of vascular collagen accumulation in right ventricular function - via pressure- volume measurements in the right ventricle of live mice in situ. 4. To examine the pulmonary vascular hemodynamics and right ventricular function of patients with pulmonary hypertension, focusing on those with excessive collagen accumulation. The successful completion of this project will yield important insights into the progression of, treatment for and recovery from pulmonary hypertension. Furthermore, these studies will answer critical questions regarding the mechanical mechanisms of right ventricular dysfunction secondary to pulmonary vascular remodeling in pulmonary hypertension.